Ferritic stainless steel (steel sheet), which is inexpensive and highly corrosion resistant, are used in a wide variety of applications including building material, transportation equipment, home electric appliances, kitchen instruments, automobile parts, etc., and the range of applications has seen further expansion in recent years. To be suitable for these applications, ferritic stainless steel is required to have not only corrosion resistance but also sufficient formability allowing the steel to be worked into desired shapes (in other words, the elongation needs to be large (hereinafter having sufficiently high elongation may be referred to as having ductility) and the average Lankford value (hereinafter may be referred to as an “average r-value”) needs to be excellent) and excellent ridging resistance. Having excellent surface properties is also required if the applications require aesthetically appealing surfaces.
In this respect, Patent Literature 1 discloses a ferritic stainless steel having excellent formability and ridging resistance, the ferritic stainless steel containing, in terms of % by mass, C: 0.02% to 0.06%, Si: 1.0% or less, Mn: 1.0% or less, P: 0.05% or less, S: 0.01% or less, Al: 0.005% or less, Ti: 0.005% or less, Cr: 11% to 30%, and Ni: 0.7% or less, and satisfying 0.06≤(C+N)≤0.12, 1≤N/C, and 1.5×10−3≤(V×N)≤1.5×10−2 (C, N, and V respectively represent the contents of the respective elements in terms of % by mass). According to Patent Literature 1, however, box annealing (for example, performing annealing at 860° C. for 8 hours) must be performed after hot rolling. This box annealing process requires about a week to finish if heating and cooling steps are also counted, and thus the productivity is low.
Patent Literature 2 discloses a ferritic stainless steel having excellent workability and surface properties, obtained by hot rolling a steel containing, in terms of % by mass, C: 0.01% to 0.10%, Si: 0.05% to 0.50%, Mn: 0.05% to 1.00%, Ni: 0.01% to 0.50%, Cr: 10% to 20%, Mo: 0.005% to 0.50%, Cu: 0.01% to 0.50%, V: 0.001% to 0.50%, Ti: 0.001% to 0.50%, Al: 0.01% to 0.20%, Nb: 0.001% to 0.50%, N: 0.005% to 0.050%, and B: 0.00010% to 0.00500%, annealing the resulting hot-rolled sheet in a box furnace or a continuous furnace of an annealing and pickling line (AP line) in a ferrite single-phase temperature region, and performing cold rolling and cold-rolled-sheet annealing. However, if a box furnace is used (box annealing), there is a problem of low productivity as with Patent Literature 1 described above. Although Patent Literature 2 makes no mention about elongation, annealing a hot-rolled sheet in a continuous annealing furnace in a ferrite single-phase temperature region results in insufficient recrystallization due to low annealing temperature, and the elongation is decreased compared to when box annealing is performed in a ferrite single-phase temperature region. Moreover, in general, when ferritic stainless steel such as one described in Patent Literature 2 is casted or hot-rolled, crystal grain groups (colonies) that have similar crystal orientations are formed and a problem of ridging arises after forming.